1. Field of the Invention
The present invention relates to a structure of an active matrix type flat panel display device incorporating peripheral drive circuits therein.
2. Description of the Related Art
Heretofore, there has been known an active matrix type liquid crystal display device incorporating peripheral drive circuits therein. This has a structure in which an active matrix circuit constituting pixel regions formed of thin film transistors (abbreviated as a TFT) and peripheral drive circuits for driving this active matrix circuit, which are also formed of thin film transistors, are integrated on a glass substrate (or a quartz substrate).
For example, in a Video Graphic Array (VGA) panel, about 300,000 thin film transistors are integrated on the same glass substrate or quartz substrate. Also, in the case of an Engineering Work Station (EWS) panel, about 1,300,000 thin film transistors are integrated on the same glass substrate or quartz substrate.
In the above structure, if one of the thin film transistors is defective, a dot defect or a linear defect is formed.
The performance of a display device is judged visually. Therefore, when the above dot defect or linear defect is present, the display device is judged as a defective product.
When a glass substrate or a quartz substrate is used, especially, the problem of breakdown by static electricity (electrostatic breakdown) is actualized because the insulating property of the substrate is high and its area is large.
For instance, in the formation of a liquid crystal panel, a plasma using process is frequently used in the formation of various thin films and etching. In the plasma using process, pulse-form static electricity is generated as will be described hereinafter. Also, a process where static electricity is generated is existent such as a rubbing process other than the above plasma process.
As described above, in the formation of thin films constituting thin film transistors and etching, a plasma process typified by a plasma Chemical Vapor Deposition method and a plasma etching method is frequently used. However, since the insulating property of a substrate used is high, there occurs such a phenomenon that discharge takes place locally in this plasma process.
A failure which is considered to be caused by this discharge occurs. Stated more specifically, the operation failure of a thin film transistor which is considered to be caused by various electrostatic breakdowns or static electricity occurs. The failure is the major cause of a reduction in the production yield of an active matrix type liquid crystal display device and other active matrix type flat panel display devices.
As the result of analyzing some examples of the occurrence of the above failures, the inventors of the present invention have reached the following findings.
Firstly, there are roughly classified into two different types of the occurrences of failures caused by static electricity or application of localized high voltage.
The first type of occurrences are caused by electrostatic pulses. The failures caused by the electrostatic pulses include a contact failure and the dielectric breakdown of an Insulating film.
The contact failure is caused by the following mechanism. Firstly, at the time of forming a thin film by a plasma CVD method or plasma etching by an Reactive Ion Etching method, localized discharge occurs. This discharge is caused by such a factor that a sample using an insulating substrate has a large area and a state where localized discharge is liable to occur is established and further such minor factors as the uneven surface of a pattern, the difference of pattern area, the slight difference of film quality, the presence of particles and the like.
As the result of the above localized discharge, high voltage is instantaneously applied to an extremely small specific region. At this point, voltage is locally induced in part of wiring and an electrostatic pulse is generated. This electrostatic pulse is generated instantaneously and a leading value of this induced voltage is extremely large.
A large current flows through a contact portion between a thin film transistor and wiring (or electrode) due to this electrostatic pulse. The instantaneous flow of a large current causes the contact portion to generate heat at a high temperature. Thereby, the contact is broken. The breakdown of this contact is permanent and is generally difficult to be repaired.
Further, the breakdown of an insulating film is due to the fact that a localized strong field is applied to the insulating film which must retain an insulation function and the insulating property of that applied portion is broken by the instantaneous flow of a large current caused by electrostatic pulses through wiring and electrodes. The breakdown of this insulating property is also permanent and is generally difficult to be repaired.
The second type of occurrences are caused by the generation of static electricity induced by plasma. This is caused by nonuniformity (such as area difference or level difference) in the shape of a wiring pattern on the substrate in the plasma using process such as film formation or etching. In this process, a localized potential difference is instantaneously induced between patterns during plasma discharge.
This localized potential difference causes localized discharge between conductive patterns or between a conductive pattern and an insulating substrate. This results in damage to a junction (such as a PI junction or NI junction) of a thin film transistor, whereby the thin film transistor malfunctions.
The damage to the junction of the thin film transistor by this localized discharge may be repaired by a heat treatment. Therefore, the failure in this case can be considered as semi-permanent.
It is therefore an object of the invention to provide a technology for improving the production yield of a liquid crystal panel by suppressing the occurrence of a failure caused by the above electrostatic breakdown.
The present invention is predicated upon the result of the above analysis. The present invention has basically two aspects. According to a first aspect of the present invention, there is provided means for suppressing the generation of electrostatic pulses. According to a second aspect of the present invention, there is provided means for suppressing the generation of static electricity induced by plasma.
In the present invention, to suppress the generation of electrostatic pulses, there are arranged protective capacitors for absorbing instantaneous electric pulses around a liquid crystal panel.
In the manufacturing process of an active matrix type flat panel display device typified by a liquid crystal panel, there is arranged wiring called xe2x80x9cshort ringxe2x80x9d to eliminate a potential difference between wiring patterns. This short ring is separated from a circuit in the end in the step of manufacturing a finished product, the short ring has no wiring function any longer.
One electrode of the above-described capacitor (protector capacitor) for absorbing electric pulses is connected to this short ring. That is, electric pulses induced by this short ring are absorbed by the above protective capacitor.
The short ring is connected to all source lines and gate lines constituting an active matrix circuit. Therefore, even if an electric pulse enters somewhere in the active matrix circuit, it is absorbed by the above protective capacitor. Even if the electric pulse is large and is not completely absorbed by the protective capacitor, its influence can be weakened.
Generally speaking, the short ring is not connected to all the gate electrodes of thin film transistors constituting peripheral drive circuits for driving the active matrix circuit. However, when the protective capacitor is arranged in an area near the peripheral drive circuit block, it can absorb electric pulses from the outside and prevent the electric pulses from going into the peripheral drive circuits. Further, the electric pulses which enter the peripheral drive circuits can be weakened.
In the present invention, as means for preventing static electricity induced by plasma, a discharge pattern for discharging this static electricity is arranged between the short ring and the active matrix circuit region and between the short ring and the peripheral drive circuit region.
According to the analysis of the inventors of the present invention, static electricity induced by plasma is easily generated mainly from a conductive pattern having a large area.
A conductive wiring pattern having the largest area in the manufacturing process of a liquid crystal panel is the short ring. That is, the short ring is used to eliminate a potential difference between wirings and suppress unnecessary discharge. On the other hand, the short ring causes the generation of static electricity by itself.
When voltage is induced by the short ring, static electricity is locally generated.
To cope with this, in the present invention, a discharge pattern (called xe2x80x9cguard ringxe2x80x9d) is provided between the short ring and the active matrix circuit region and between the short ring and the peripheral drive circuit region to cancel static electricity induced by plasma.
That is, before static electricity induced by the short ring enters the active matrix circuit region or the peripheral drive circuit region, this static electricity is discharged in the step where it passes through the discharge pattern.
Alternatively, in a region which is affected by a potential difference, a discharge pattern is provided between the short ring and the circuit. Thus, the circuit is prevented from being affected by a potential difference which is produced between the circuit and the short ring.
Thus, it is possible to suppress damage to the thin film transistors arranged in the active matrix circuit region and the peripheral drive circuit region caused by static electricity induced by the short ring.
One aspect of the present invention, as shown by an embodiment thereof in FIG. 1, is a display device comprising an active matrix circuit 108 and peripheral drive circuits 104 and 105 for driving the active matrix circuit, arranged on the same substrate 101, characterized in that the active matrix circuit 108 and peripheral drive circuits 104 and 105 are surrounded by discharge patterns 112, 103 and 106.
In the above configuration, it is advantageous that the pitch of the discharge patterns is smaller than the pitch of pixels of the active matrix circuit.
This is intended to prevent discharge from being produced in the active matrix circuit by an electric pulse which enters the active matrix circuit.
Another aspect of the present invention, as shown by a manufacturing process according to an embodiment in FIG. 3, is an active matrix type display device comprising an active matrix circuit (pixel region) arranged on the same substrate 301 and a capacitor formed adjacent to the active matrix circuit, characterized in that the capacitor comprises an electrode 307 formed in the same layer and from the same material as a gate electrode 310 of a thin film transistor arranged in the active matrix circuit, an insulating film 306 made from a material forming the gate insulating film of the thin film transistor, under the electrode, and a semiconductor film 302 constituting the active layer of the thin film transistor under the insulating film 306.
Another aspect of the present invention, as shown by an embodiment in FIG. 3, is a method for manufacturing a display device comprising an active matrix circuit (pixel region) and peripheral drive circuits for driving the active matrix circuit, arranged on the same substrate 301, characterized by comprising the steps of:
forming a short ring 307 to be connected to all the gate lines and all the source lines constituting the active matrix circuit;
forming impurity regions 319 to 321 and 300 for thin film transistors arranged in the active matrix circuit by the implantation of impurity ions; and
forming a capacitor in the short ring region by implanting impurity ions into a semiconductor layer 302 under the short ring using the short ring 307 as a mask simultaneously with the above step.
In the above constitution, the capacitor is formed in a region where the electrode (short ring) 307 and a semiconductor area 322 face each other through the insulating film 306. This capacitor functions as a protective capacitor for absorbing an electric pulse.
Another aspect of the present invention, as shown by an embodiment in FIG. 3, is a method for manufacturing an active matrix type display device, characterized by comprising the steps of forming a thin film semiconductor layer 302 under the short ring 307, and forming a capacitor using the thin film semiconductor layer 302.
Another aspect of the present invention, as shown by an embodiment in FIG. 3, is a method for manufacturing an active matrix display device, characterized by comprising the step of forming a capacitor using a thin film semiconductor layer 302 which is present under the short ring 307 simultaneously with the step of forming impurity regions 319 to 321 and 300 for thin film transistors arranged in the active matrix circuit.